(1) Field of the Invention
The invention relates to a method for comminution of mill feed, in particular for the cement industry as well as the steel industry, ore industry, coal gasification plants for the power plant industry and chemical industry.
The invention is provided in particular for clinker/slag grinding plants in which the grinding material supplied must be subjected to a grinding-drying process. The invention is also suitable for raw powder grinding plants of the cement industry.
(2) Description of Related Art
In cement/granulated blast furnace slag grinding plants of the cement industry which are operated in combination with a kiln line system of the clinker burning process, exhaust gases formed in a cement rotary kiln are generally available as a heat source for drying the grinding material.
DE 198 36 323 C2 describes composite grinding plants with air swept roller mills for grinding-drying processes.
Grinding-drying plants without external sources, such as for example rotary kiln exhaust gases or clinker cooler gases, require suitable devices, for example burners, in particular hot gas generators, to produce a hot process gas, for example hot air, which is fed to a mill for drying the moist grinding material and removing the quantity of water from the grinding circuit. The term “hot gas” or “process gas” is intended below to also include air or hot air and the term “fresh air” is always to include also fresh gases from other processes which are for example at ambient temperature.
For coal grinding plants in the ore industry, the metallurgical industry (blast furnace process) and coal gasification (power plants and chemical industry) there are in the majority of cases no external energy sources available for the drying process in the mill.
In order to guarantee drying during the grinding in a vertical air swept mill, for example an air swept roller mill, and to avoid falling below the dew point of the process gas in the mill circuit, a process gas temperature downstream of a classifier of approximately 80 to 100° C. is generally provided in a cement/granulated blast furnace slag grinding plant. The hot gases supplied in the mill must be dry enough to be able to absorb the moisture contained in the grinding material. For this reason it is not possible to convey the process air in a closed circuit but instead ambient air and/or dry hot gases must be constantly fed to the grinding circuit from externally. The supplied ambient air and/or the supplied hot gases must be discharged again at the flue together with the absorbed material moisture. Due to this type of process there is a large heat loss flow, as the gases removed at the flue have a temperature of around 80 to 110° C. and the heat content thereof cannot currently be further used in the grinding system.
The ambient air fed to the grinding system consists on the one hand of portions which are fed in a controlled way to the system, for example as combustion air or fresh air, and of portions which penetrate the system in an uncontrolled way due to leakages in the system (false air).
In particular in granulated blast furnace slag grinding plants a relatively large volume flow must be removed at the flue in order to remove the moisture contained in the feed material from the system. Granulated blast furnace slag has after granulation with water a moisture content of around 30%. After preliminary water removal, the moisture content of the granulated blast furnace slag which is fed to a vertical air swept mill can still be up to 15%.
The heat energy fed to the process gas or the hot air using a burner, for example a hot gas generator, is required for a large part for vaporisation of the water present in the grinding material supplied. Besides this, heat energy is required in order to heat the false air undesirably entering the grinding system from the ambient temperature to the mill outlet temperature, as the penetrated false air must be removed via the flue at this temperature. A portion of the supplied heat energy is also necessary in order to heat the combustion air required for the burner and the other fresh air fed to the system (ambient air) to the mill outlet temperature in order to be able to discharge this to the atmosphere again via the flue.
It is an object of the invention to create a method for comminution of material to be ground, in particular granulated blast furnace slag, a granulated blast furnace slag/cement mixture or a cement/additive mixture, which has an improved energy balance and thereby lower operating costs.